What is an Analog to Digital Converter?

An analog-to-digital converter, or A / D converter, or ADC for short, usually refers to an electronic component that converts an analog signal into a digital signal. A typical analog-to-digital converter converts an input voltage signal into an output digital signal. Because the digital signal itself has no practical significance, it only represents a relative size. Therefore, any analog-to-digital converter needs a reference analog quantity as the conversion standard. The more common reference standard is the largest convertible signal size. The digital output indicates the magnitude of the input signal relative to the reference signal.

Analog-to-digital converter

A circuit that converts analog signals into digital signals is called an analog-to-digital converter (referred to as a / d converter or adc, analog to digital converter). It is converted into a digital signal with discrete time and discrete amplitude. Therefore, the A / D conversion generally goes through four processes: sampling, holding, quantizing, and encoding. In actual circuits, some of these processes are combined, for example, sampling and holding, quantization and encoding are often implemented simultaneously during the conversion process.

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The analog-to-digital conversion usually goes through the steps of sampling, holding, quantizing, and encoding. Sampling theorem: When the sampling frequency is greater than twice the highest frequency component of the analog signal, the sampled value can reflect the original analog signal without distortion ^[2]

There are many types of analog-to-digital converters. According to different working principles, they can be divided into indirect ADCs and direct ADCs. ^[2]

In analog-to-digital converters, the operating conditions can be improved by introducing a dither signal (Dither). Dither signal is a small amount of random noise mixed into the input signal before conversion (

The following three methods are mainly introduced: successive approximation method, double integration method, and voltage-frequency conversion method

1) Successive approximation

Successive approximation A / D is a more common type of A / D conversion circuit, and the conversion time is in the order of microseconds.

The A / D converter using the successive approximation method is composed of a comparator, a D / A converter, a buffer register, and a control logic circuit, as shown in the figure.

Example 1: For a 2-bit voltage analog-to-digital converter, if the reference is set to 1V, the output signal has 00, 01, 10, 11, and 4 codes, which represent the input voltage between 0V-0.25V, 0.26V -0.5V, 0.51V-0.75V, 0.76V-1V corresponding input. Divided into 4 levels of coding, when a 0.8V signal is input, the data output by the converter is 11.

Example 2: For a 4-bit voltage A / D converter, if the reference is set to 1V, the output signals are 0000, 0001, 0010, 0011, 0100, 0101, 0110, 0111, 1000, 1001, 1010, 1011 1100, 1101, 1110, 1111, 16 kinds of codes, which respectively represent the input voltage between 0V-0.0625V, 0.0626V-0.125V, ........... 0.9376V-1V. Divided into 16 levels of coding (more accurate) When a 0.8V signal is input, the data output by the converter is 1100.

Conversion accuracy

(1) Resolution

The resolution of the A / D converter is expressed in the number of bits of the output binary (or decimal) number. It illustrates the A / D converter's ability to resolve input signals. In theory, the A / D converter with n-bit output can distinguish between two different levels of input analog voltage, and the minimum value of the input voltage can be 1 / 2n of the full-scale input. When the maximum input voltage is constant, the more the output digits, the higher the resolution. For example, the output of the A / D converter is an 8-bit binary number, and the maximum value of the input signal is 5V. Then the minimum voltage of the input signal that this converter can distinguish is 9.53mV.

(2) Conversion error

The conversion error is usually given as the maximum value of the output error. It indicates the difference between the actual digital output of the A / D converter and the theoretical output digital. It is usually expressed in multiples of the least significant bit. For example, given the relative error ± LSB / 2, this indicates that the error between the actual output digital value and the theoretical output digital value should be less than the least significant half word.

2 Conversion time

The conversion time refers to the time elapsed since the A / D converter started to convert the control signal to the stable digital signal at the output. The conversion time of the A / D converter is related to the type of conversion circuit. Different types of converters have very different conversion speeds. The parallel comparison A / D converter has the highest conversion speed. The conversion time of a single-chip integrated A / D converter with 8-bit binary output can reach within 50ns, followed by the successive comparison A / D converter. Most of them have a conversion time of 10 Within ~ 50s, the speed of the indirect A / D converter is the slowest. For example, the conversion time of the double-integral A / D converter is mostly between tens of milliseconds and hundreds of milliseconds. In practical applications, the selection of the A / D converter should be comprehensively considered from the total number of system data bits, accuracy requirements, range of input analog signals, and input signal polarity.